Damp Proofing
Across London
Rising damp and penetrating damp diagnosed properly, then treated with the right system for the construction you actually have — not a default package sold to every property on the street.
What Damp Proofing Actually Means
"Damp proofing" gets used as a catch-all, which is part of why so much of it goes wrong. In practice it covers several distinct defects with different causes and different fixes. Get the diagnosis right and the repair is usually straightforward. Get it wrong and you pay twice — once for the treatment that did not work, and again for the one that does.
The most common mistake
A wall is damp, so a chemical damp proof course gets injected. But the actual cause was a raised garden path bridging the existing DPC, or a leaking downpipe, or a failed gutter joint. The injection does nothing, because there was never a problem with the damp proof course. This is the single most frequent reason we get called to properties that have already been "treated".
Rising Damp
Rising damp is groundwater drawn up through the masonry by capillary action, in properties where the damp proof course is absent, bridged or has failed. It is genuinely less common than it is sold, but it is real, and in London's older housing stock — particularly solid-wall Victorian and Georgian property — we do find it.
What it looks like: a horizontal tide mark up to roughly a metre above floor level, salt deposits (efflorescence) on the plaster surface, blown or hollow-sounding plaster, decaying skirtings, and a reading profile that decreases as you go up the wall. It does not come and go with the weather.
How we treat it: after confirming the diagnosis with drilled masonry samples and salts analysis, we install a remedial damp proof course — normally a low-pressure injected cream or fluid silane/siloxane system — then remove the contaminated plaster and replace it with a salt-inhibiting render system. That second half matters. Hygroscopic salts left in old plaster will keep pulling moisture out of the air for years and make a correctly installed DPC look like a failure.
Penetrating Damp
Penetrating damp is water coming in laterally through the building envelope — through defective pointing, spalled brickwork, a cracked render coat, failed sealant around a window, a blocked cavity, a bridged wall tie or, very often, rainwater goods that have quietly stopped doing their job.
What it looks like: damp patches at any height, often well above the ground floor, that get noticeably worse after driving rain and dry back in settled weather. Staining frequently mirrors something outside — a downpipe, a sill, a chimney stack.
How we treat it: we fix the ingress route first. That might be repointing, localised brick replacement, render repair, flashing or sealant work, or clearing and repairing rainwater goods. Only then do we deal with the internal damage. Where a solid wall faces severe driving rain exposure, a breathable water-repellent treatment or an internal membrane may be specified — but as a supplement to the repair, never instead of it.
The Treatments We Use
| System | Used for | Typical guarantee |
|---|---|---|
| Injected DPC (cream) | Confirmed rising damp in solid or cavity masonry | 20 years |
| Salt-inhibiting render | Reinstating walls after DPC works or salt contamination | 10 years |
| Cavity drain membrane | Persistently wet walls, below-ground and retained walls | 10–20 years |
| Cementitious tanking | Cellars, vaults, lift pits and other contained structures | 10–20 years |
| External repair & repointing | Penetrating damp through a defective envelope | Workmanship guarantee |
Guarantee periods depend on the specific product, substrate and specification. The exact terms are set out in your survey report before you commit to anything.
What It Costs
Honest answer: it depends entirely on what is wrong, and anyone quoting a firm price before seeing the property is guessing. As a rough guide for London work, a single-wall damp proof course with replastering typically runs from around £900–£1,800, a full ground-floor treatment in a mid-terrace from around £2,500–£5,000, and localised penetrating damp repairs from a few hundred pounds. Your survey report gives you an itemised, fixed price rather than a range.
How Long It Takes
Most residential damp proofing is a two to five day job on site. Replastered walls then need time to dry before redecoration — allow roughly one month per 25mm of thickness, which in practice means four to six weeks before you paint, and a breathable mist coat first. We put the drying times in writing, because decorating too early is the most common cause of avoidable disappointment after otherwise good work.
Damp ProofingFAQs
It is real, but it is diagnosed far more often than it occurs. The majority of "rising damp" we are called to inspect turns out to be penetrating damp, condensation, a plumbing leak or a bridged damp proof course. That is exactly why we test before we treat — salts analysis and a moisture profile up the wall will distinguish genuine rising damp from the alternatives in a way that a handheld conductivity meter on its own cannot.
In almost all cases, yes. Where rising damp has been present, the existing plaster is contaminated with hygroscopic nitrate and chloride salts that absorb moisture from the air. Leave it in place and the wall will keep reading damp and keep showing tide marks, however well the new DPC works. Removing the affected plaster and replacing it with a specified salt-inhibiting render is part of the treatment, not an optional extra.
Yes, but the specification changes. Solid-wall period buildings were designed to breathe, and sealing them with dense cement renders or impermeable coatings often moves the problem rather than solving it. In listed and conservation-area property we favour breathable lime-based systems, external repair and improved evaporation, and we will tell you where consent is likely to be needed before any work starts.
Our guarantees are assignable to a new owner. You will be given the guarantee documentation on completion along with the survey report and specification — keep them together, because buyers' solicitors routinely ask for both, and having them to hand removes a common conveyancing delay.
Get the Cause Found & Fixed
Book a survey and you get a written report: what is causing the damp, what it takes to fix it and what it costs. No pressure, no guesswork.
Survey fee deducted from the cost of works if you proceed